diff --git a/src/lib/__tests__/graphUtils.test.ts b/src/lib/__tests__/graphUtils.test.ts
index 7c0c3aa..64b2551 100644
--- a/src/lib/__tests__/graphUtils.test.ts
+++ b/src/lib/__tests__/graphUtils.test.ts
@@ -208,7 +208,7 @@ describe("graph traversal helpers", () => {
     expect(affectedEdges).toHaveLength(4);
   });
 
-  it("detects cycles and marks nodes with errors", async () => {
+  it("detects cycles and returns true without mutating input nodes", async () => {
     const { checkForCyclesAndMarkErrors } = await loadGraphUtils();
     const nodes = [createNode("x"), createNode("y")];
     nodes.forEach((n) => (n.data.dirty = false));
@@ -217,9 +217,10 @@ describe("graph traversal helpers", () => {
     const hasCycle = checkForCyclesAndMarkErrors(nodes, edges);
 
     expect(hasCycle).toBe(true);
+    // input nodes must not be mutated – caller owns state updates
     nodes.forEach((node) => {
-      expect(node.data.error).toBe(true);
-      expect(node.data.extendedError).toMatch(/Cycle detected/);
+      expect(node.data.error).toBeUndefined();
+      expect(node.data.extendedError).toBeUndefined();
     });
   });
 });
diff --git a/src/lib/graphUtils.ts b/src/lib/graphUtils.ts
index 436c3ca..043f47c 100644
--- a/src/lib/graphUtils.ts
+++ b/src/lib/graphUtils.ts
@@ -448,7 +448,7 @@ export function mergePartialResultsIntoFullGraph(
  * Flags cycles instantly so the user doesn’t have to wait for the API
  * to complain.
  *
- * @returns `true` if a cycle was found (and nodes were marked with errors)
+ * @returns `true` if a cycle was found
  */
 export function checkForCyclesAndMarkErrors(
   affectedNodes: Node<CalculationNodeData>[],
@@ -479,14 +479,6 @@ export function checkForCyclesAndMarkErrors(
     }
   }
 
-  /* --- mark error if not all processed --------------------------------------- */
-  const hasCycle = processed !== affectedNodes.length;
-  if (hasCycle) {
-    affectedNodes.forEach((n) => {
-      n.data.error = true;
-      n.data.extendedError =
-        "Cycle detected in this sub-graph – calculation aborted.";
-    });
-  }
-  return hasCycle;
+  /* --- detect cycle if not all nodes were processed -------------------------- */
+  return processed !== affectedNodes.length;
 }